View (G) shows a plug from an engine that ran
dwell. It should not have changed more than 3 degrees.
normally. The damage to the plug was caused by the
If you note a greater change, the distributor has worn
mechanic attempting to bend the center electrode.
bearings or a worn cam and should be removed for
View (H) shows a plug from an engine that had a
SPARK PLUGS.--When plugs receive periodic
cleaning and gaping, they will function better and last
The plugs shown in views (F), (G), and (H) cannot
longer. Spark plug wires should be carefully removed
be reused, but the others can be reconditioned.
by grasping them close to their terminals for the pull.
Before you attempt to clean a plug, remove oily
Do not jerk them from the spark plug terminal. Loosen
deposits with an approved solvent. This is to prevent the
each plug one or two turns, and then use low-pressure
cleaning material from soaking and packing into the
compressed air to blow out any dirt around the spark
area around the center electrode insulator.
plug hole. This prevents foreign matter from entering
A machine of the type shown in figure 6-53 is used
to clean (sand blast) and test spark plugs.
Following the manufacturer's operating instruc-
tions, clean the plugs, blasting only long enough to
Always wear safety goggles when working
remove the deposits. Prolonged blasting damages the
with compressed air.
center electrode insulator.
Once cleaning is completed, the electrodes must be
Remove the plugs and ensure that the gaskets
filed clean and square. The machine blast does not
(where used) are also removed. As you remove the
clean this area. If the electrodes are not filed clean, the
plugs, keep them in order so that any peculiar condition
required voltage remains high and the plug may
of any plug can be related to a specific cylinder.
The plugs shown earlier in figure 6-49 illustrate
Use a fine cut point file to file the end of the center
some of the plug conditions that may be encountered.
electrode flat. This produces clean, sharp edges that
The cause for each is discussed below.
improve plug performance. Remove only enough metal
to clean and square the electrodes.
View (A) shows a plug from a mechanically sound
engine, running at the correct temperature. Some
Use a round wire gauge (not a feeler gauge). Set the
deposits, light tan or gray in color, are present, but there
gap to the manufacturer's specifications by bending the
is no evidence of burning. Some gap growth has
occurred, but not in excessive amounts.
Testing spark plugs outside the engine can be very
View (B) shows a plug from an engine having
misleading. There is no true relationship between firing
excessive combustion chamber deposits. This is most
prevalent in engines operated at slow speeds and in
View (C) shows a plug from a cold running engine.
The deposits are unburned fuel.
View (D) shows a plug from an engine that was
running too hot due to over-advanced timing or to
cooling system blockage.
View (E) shows a plug from an engine using
excessive amounts of oil. The plug is drowned in oil
that was bypassed through the rings or valve guides.
View (F) shows a plug from an engine with severe
preignition. Preignition can be caused by an overheated
plug, a piece of glowing carbon, a hot valve edge, etc.
The damage shown is a result of temperatures in the
combustion chamber in excess of 2,700F.
Figure 6-53.--Spark plug cleaning and testing machine.